The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
In conventional induction motor drives the stator neutral is inaccessible and the stator winding is excited only from one end by a single three phase supply. In an open-ended induction motor drive, the stator neutral of the induction motor is opened and the stator winding is connected to another three phase supply at an end opposite the first-mentioned three phase supply. Thus, two sets of voltages excite the stator winding of the induction motor from both ends. Commonly, each three phase supply is derived from a two-level inverter. In consequence the stator is excited from two two-level inverters from both ends, which is equivalent to the induction motor being driven from a three-level inverter. In order to annul any zero sequence voltage induced circulating currents, appropriate measures are taken i.e. using a common mode filter or using an isolated DC-link for the two inverters. The three-level inverter has some advantages compared to the two-level inverter in terms of switching ripple and switching losses.
Although common, these systems have significant disadvantages. In particular, power for the motor originates typically from a three phase AC source such as a utility. In order to create voltages suitable for driving the motor, the power from the three phase AC source is converted to a DC source of power. The DC source of power is then used by inverters to generate suitable voltages to drive the motor. In view of this, the motor control circuit must include a converter to convert the three phase power to DC, a suitable substantial storage device (e.g. large capacitor) to temporarily store the DC power as well as an inverter to use the DC power and selectively energize the windings to drive the motor. In addition to its expense, a large storage capacitor adds additional weight, can be a reliability problem and is difficult to integrate into either the motor or the inverter. The storage capacitor also creates an inrush-current problem when the system is switched on as well as cause additional currents under input imbalance conditions. Furthermore, the peak motor phase voltage is equal to the peak value of the utility phase voltage.
A similar situation exists with power transformers. As is known, a common power transformer (e.g. 50 or 60 Hz) includes three single-phase transformers or one three-phase transformer. The disadvantage of such a transformer is its size and weight due to its low operating frequency.
In order to overcome the limitation of conventional low frequency power transformers, power electronic transformers based on voltage-link converters have been investigated. However, like the motors and generators discussed above, the configurations require a large dc-link capacitor that is bulky, unreliable and problematic at high temperatures for use with electronic switching devices such as SiC devices. Furthermore, in view of the multiple conversion stages present in these systems (i.e. AC to DC, DC to HF (high frequency) AC, HF AC to DC and DC to AC) efficiency is reduced.